Storage batteries, such as are used in vehicles and stationary equipment, may be relatively large in size. In contrast, batteries used in appliances, lighting devices, watches and the like may be relatively small. The present invention finds application across the size range of rechargeable batteries from small button batteries to large industrial batteries.
The oldest and best known type of rechargeable battery is that known as the lead-acid battery. While the present invention is not so limited, it has been developed as an improved lead-acid type battery. Accordingly, the description is primarily in terms of such a battery.
A typical lead-acid battery comprises a positive electrode, a negative electrode, one or more separators, and an electrolyte. The electrodes are commonly coated lead or lead alloy grids. They function both as electrical contacts and as mechanical load-bearing elements.
A separator may be any porous, perforated, or fibrous sheet that sufficiently isolates the electrodes to prevent short circuiting. However, the separator must also be sufficiently open to permit ion transfer through the electrolyte contained in the separator.
Perforated plastic, or glass fiber, sheets are commonly used as separators. A compressed mat of glass fibers is currently used in many commercial storage batteries. However, porous earthenware and sintered silicate sheets have also been proposed.
The electrolyte may be any ionizable liquid that can provide ion transtar between the electrodes. In a lead-acid battery, sulfuric acid is the electrolyte commonly employed.
A battery may be packaged in a plastic case for insulating purposes. However, the electrodes constitute the primary mechanical support and load-bearing means in current storage battery construction.
The glass fiber mat, now in use as a separator, has certain desirable features. It readily takes up and holds electrolyte, a property commonly referred to as wettability or wickability. It is also resistant to attack by the electrolyte, and provides acceptable electrical properties.
The fiber mat separator is, however, flexible and lacking in mechanical strength. This means that the electrodes, the casing, or other support members must be the primary source of structural integrity in a battery.
Consequently, a rigid, strong, light-weight separator, also having the desirable features of the glass fiber separator, would be a boon. It would provide structural support, facilitate automated production, and, depending on battery design and separator thickness, could reduce weight.
A factor in battery life is the tendency of material, e.g. lead compounds, to flake off an electrode during use of the battery. This undesirable occurrence is prevented to some extent by the compact battery assembly where the separator is compressed between the electrodes. However, since either a glass fiber mat, or a polymer separator, is flexible, it may still distort and permit electrode disintegration. A rigid separator would avoid this undesirable occurrence.
A rigid, relatively strong separator would also reduce manufacturing costs by permitting automated operations. Heretofore, efforts to automate battery construction have been hampered by the lack of rigidity in the glass fiber separator. Consequently, battery assembly has remained a manual operation to a large extent.
It is a major purpose of our invention to provide an improved battery construction embodying a rigid, porous separator. It is a further purpose to provide a battery that exhibits more consistent operating properties, and that has a longer life time than currently available batteries. Finally, it is a purpose to provide such an improved battery with a simple construction that lends itself to automated assembly.